This invention relates to a method and apparatus for transmitting digital data between digitally operating subscriber stations and a subscriber concentrator of a time division multiplex (TDM) telecommunication network. In particular, the invention is concerned with such data transmission carried out via two-wire lines utilized on a four-wire basis through a time-interleaved arrangement of transmit and receive modes of a time hybrid circuit.
The digitalization of TDM telecommunication networks, insofar as subscriber stations are concerned, has, among other things, the advantage that subscriber concentrators can be designed more simply.
As a rule, four-wire lines are needed in TDM systems in which the data are transmitted in digital form. That means, however, that the above mentioned advantage of the digitalization of the subscriber stations is negated by an additional expenditure caused by the four-wire design of the subscriber lines connecting the subscriber stations to a subscriber concentrator.
For the information exchange between subscriber stations and the subscriber concentrator the use of two-wire lines has been disclosed in the prior art (West German patent application No. P 23 46 984.3-31), but in the prior art apparatus the subscriber stations do not operate digitally and are connected over time hybrid circuits to the subscriber concentrator. Due to the interleaving on a time-division basis of send and receive mode, time hybrid circuits are utilized on a four-wire basis.
It is an object of the invention to provide a means and method of operating such time hybrids which avoids interference with the data transmission by echo signals and interaction between the time hybrids allocated to the subscriber stations and those assigned to the concentrator.